ath10k: remove pci features var

The ATH10K_PCI_FEATURE_MSI_X was originally
introduced to support both chips QCA988Xv1 and
QCA988Xv2. Since v1 isn't supported anymore it
doesn't make sense to keep the feature flag
around. Since this is the last one remove the
whole thing.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>